1. Field of the Invention
The present invention relates to oriented films with improved attributes prepared using impact copolymer polypropylene. The present invention particularly relates to biaxially oriented films with improved attributes prepared using an impact copolymer polypropylene.
2. Background of the Art
Impact polypropylene copolymers are commonly used in a variety of applications from automobile parts, household appliances, toys, batteries, and packaging where properties such as strength, electrical insulation, and impact resistance are desired. Commercial grades of these polymers are available in a wide range of physical properties including melt flows, impact strengths and molecular weight distributions.
Impact polypropylene can be prepared by mechanically admixing a polypropylene homopolymer and a rubber. Commonly, the two components are admixed in the form of pellets and then melt blended, extruded, and pelletized. Such a process, while comparatively inexpensive, does not produce an impact polypropylene having the same properties as an impact polypropylene copolymer.
Impact polypropylene copolymers are made of at least two components, a homopolymer polypropylene component and a copolymer component. These two components are typically made in sequential polymerization reaction processes. The first reaction produces the homopolymer and the second reaction produces the copolymer. The copolymer is thereby incorporated within the matrix of the homopolymer component. The homopolymer component imparts overall stiffness on the polymer, while the copolymer component imparts the rubbery characteristics as well as the desired impact resistance.
It is well known that processing variables influence the properties of the resulting impact copolymer. Manufacturing with varying ratios of the components, adding nucleating agents, and even adding a third monomer to the copolymer component are all known to the art. Each of these variables imparts unique properties to the resulting impact copolymer.
Biaxially orienting polymers or “stretching” imparts unique characteristics to polymers as well. Such stretching can occur either in the machine (longitudinal) direction, in the transverse direction or both. The resulting film is significantly decreased in thickness and is often used in applications such as carton-sealing tape, cigarette overwrap or food packaging. The level of light diffraction is low, commonly referred to as high gloss, resulting in the inability to write on the film. When the polymer is a impact polypropylene copolymer that has been filled, for example, the resulting film can be a breathable film having a water vapor permeability of greater than 17 grams per square meter per day for a 0.0025 cm film (15 g/m2·d (25 μm film) and an oxygen permeability of greater than 8000 cc·25 μm/m2·day.
In contrast to impact polypropylene copolymers, it is known to prepare biaxial films from homopolypropylene. For example, it is known in the art to form a biaxially oriented polypropylene film having a high-gloss (preferably of more than about 90), and a low haze (preferably less than 2%). Additional treatment, such as corona or flame treatment is required to achieve printability of such a film.